1. Field of the Invention
The present invention relates to radio selective call receivers and, more specifically, to a radio selective call receiver and a method for receiving a selective calling signal which changes data transmission rate during receiving.
2. Description of the Related Art
With the widespread use of selective call receivers, occupied channels form an increasing proportion of the total channels of the communications system, resulting in difficulty in ensuring a channel for calling.
To cope with such a problem, there has been proposed a bit-rate changeable receiver in Japanese Patent Laid-open No. 1-502870. The receiver is provided with a controller which controls the cut-off frequency of a low-pass filter depending on the bit rate of a received signal. More specifically, the low-pass filter is controlled such that the frequency components higher than the bit-rate frequency are cut off to eliminate noise components.
However, the inventor has found that changing the cut-off frequency of the low-pass filter produces a phase shift in the received signal passing through the low-pass filter. For example, in the case where the bit rate switches from 1600 bps (bits per second) to 3200 bps, the cut-off frequency of the low-pass filter also switches from 1600 bps to 3200 bps. In this case, the received signal shifts in phase by a predetermined time period T as shown in FIG. 4A. If such a phase shift is ignored, the data decision fails to be performed each time the received signal reaches a crest and a trough, resulting in faulty receiving.
An object of the present invention is to provide a selective call receiver and a receiving method which can achieve the reliable data decision even when the transmission rate is changed during receiving.
According to the invention, a radio selective call receiver for receiving a selective calling signal which is changeable in transmission rate is comprised of a filter for filtering out a selected frequency range of components of the selective calling signal to produce a received signal, wherein the selected frequency range is determined depending on a transmission rate of the selective calling signal. The radio selective call receiver further includes a data decision section performing data decision of the received signal according to a timing signal, and a timing generator for generating the timing signal from a reference timing signal based on a predetermined change of the received signal when a first transmission rate is changed to a second transmission rate.
Since the timing signal is generated based on the predetermined change of the received signal when a first transmission rate is changed to a second transmission rate, the data decision section can perform the data decision of the received signal according the timing signal synchronizing to the received signal, resulting in reliable receiving operation even in the case of the transmission rate being changed.